1. Field of the Invention
The invention relates to a centrifugal compressor unit. More specifically, the invention relates to a centrifugal compressor unit of the integrated type, that is to say of the type in which the compressor and a motor means for driving the compressor are mounted in a common housing sealed against the gas handled by the compressor.
2. Brief Description of the Related Art
With reference to FIG. 1, a conventional integrated compressor unit includes a motor means, generally including an electric drive motor 10 and a centrifugal compressor 12 including, for example, several compression stages, the entity being mounted in a common housing sealed against the gas handled by the compressor.
As can be seen in FIG. 1, the motor 10 rotationally drives a rotor 16 itself rotationally driving a driven shaft 18 supporting a set of compression impeller wheels 20, 22, 24 and 26.
In the example of a compressor unit illustrated in FIG. 1, the compressor includes four compression stages which together compress a gas taken in via an inlet pipe 28 to deliver it on the outlet side 30, passing via a volute casing 32.
The rotor 16 and the driven shaft 18 are connected via a flexible coupling 34. In this case, the rotor 16 and the driven shaft 18 are each supported by radial bearings 36, 38, 40, and 42. An opening 44 formed in the housing 14 and closed off by a blanking means 46 allows access to the flexible coupling 34 for assembling the compressor.
Finally, FIG. 1 shows that an axial thrust bearing 48 limits the axial displacement of the driven shaft 18, while an equalizing piston 49 allows the axial pressure applied to the driven shaft while the compression unit is in operation to be equalized.
Producing a compressor unit in the form of a motor and of a compressor which are located in a pressurized sealed common housing makes it possible to eliminate sealing gaskets needed by the compressor. These are elements liable to compromise the reliability of the compressor unit and to be the source of gas leaks into the atmosphere. In this case, the motor is located actually within the gas handled by the compressor. In order to avoid an excessive increase in the mechanical losses of the motor as a result of ventilation, the motor is arranged in such a way that it is at the intake pressure of the compressor. Thus gas is circulated through the motor in order to remove the losses, that is to say in the stator in order to remove the losses through a Joule effect in the windings, and in the air gap between the rotor and the stator in order to remove the ventilation losses and the eddy current losses in the rotor.
This is why centrifugal compressor units are generally provided with means for cooling the motor means and the guide bearings by tapping off gas leaving the first compression stage in order to cool the motor and the bearings. In this respect, reference may be made to documents EP-A-1 069 313 and U.S. Pat. No. 6,390,789, which describe various types of motor-compressor units in which the motor and the bearings are cooled by tapping cooling gas from the outlet side of the first compression stage.
However, this type of cooling technique presents a certain number of major disadvantages, particularly it does not allow optimum cooling of the motor and of the bearings. There is a need to alleviate this disadvantage and to provide a motor-compressor unit that exhibits improved cooling means.